The present application relates to heavy vehicles which have a service brakes holding function, and is particularly directed to an apparatus for setting park brakes of a heavy vehicle, such as a truck, during a failure of a service brakes holding function of the vehicle.
When a truck driver desires to apply park brakes of the truck, the driver typically operates a manually-operable parking switch located in the truck cab. A signal indicative of the driver's intent to apply the park brakes is provided in response to operation of the parking switch. The signal is applied to parking brake valves so as to exhaust air in one or more chambers of spring brake chambers which are operatively coupled to parking brake springs. When air in the spring brake chambers is exhausted to atmosphere and system air pressure drops to less than a predetermined threshold pressure, the parking brake springs are automatically applied without any human intervention to set the park brakes. Operation of the park brakes during a park braking function to park the truck is well known. It would be desirable to set the parking brakes during a vehicle function which is other than a park braking function, such as during a failure of a service brakes holding function, of the truck.